The present invention relates to a method of making cemented carbide articles using binder phase powders with spherical, non-agglomerated particles.
Cemented carbide contains mainly tungsten carbide and cobalt, often along with certain other carbides, e.g., carbides of titanium, tantalum, niobium, chromium, etc. It contains at least one hard but brittle (carbide) phase and a relatively less hard but ductile and tough metal (binder) phase, particularly cobalt. This results in materials combining hardness and toughness which have found many applications, for instance in rock drilling and metal cutting tools, wear parts, etc.
Cemented carbide is made by techniques usual in powder metallurgy, that is:
mixing the constituent powders (carbides, cobalt and possibly other hard materials) by milling, using mills (rotating ball mills, vibrating mills, attritor mills, etc.) equipped with non-polluting milling media which themselves are made of cemented carbide. The milling is made in the presence of an organic liquid (for instance ethyl alcohol, acetone, etc.) and an organic binder (for instance paraffin, polyethylene glycol, etc.) in order to facilitate the subsequent granulation operation;
granulation of the milled mixture according to known techniques, in particular spray drying. The suspension containing the powdered materials mixed with the organic liquid and the organic binder is atomized through an appropriate nozzle in the drying tower where the small drops are instantaneously dried by a stream of hot gas, for instance, a stream of nitrogen. The granules collected at the lower end of the tower have an average diameter adjustable by the choice of appropriate nozzles, between 100 and 200 .mu.m. Such granules flow easily, in contrast to fine or ultra-fine powders. The formation of granules is necessary in particular for the automatic feeding of compacting tools used in the subsequent stage;
compaction of the granulated powder in a matrix with punches (uniaxial compaction) or in a bag (isostatic compaction), in order to give the material the shape and dimensions as close as possible (considering shrinkage) to the dimension wished for the final body. If necessary, the compacted body can be subjected to a machining operation before sintering; and
sintering of the compacted bodies at a temperature and for a time sufficient to obtain dense bodies with a suitable structural homogeneity.
The sintering can equally be carried out at high gas pressure (hot isostatic pressing), or the sintering can be complemented by a sintering treatment under moderate gas pressure (process generally known as SINTER-HIP).
The sintered cemented carbides can be characterized in particular by their porosity and their microstructure (observed by optical or electron microscopy).
The cobalt powders conventionally used in the cemented carbide industry are obtained by calcining cobalt hydroxide or oxalate followed by a reduction of the oxide so obtained by hydrogen; see for instance, "Cobalt, its Chemistry, Metallurgy and Uses", R. S. Young Ed., Reinhold Publishing Corporation (1960) pages 58-59. These conventional cobalt powders are characterized by a broad particle size distribution with strongly aggregated particles in the form of agglomerates with a sponge-like aspect, which are difficult to mill since there are strong binding forces between the elementary particles in these aggregates.
In U.S. Pat. No. 4,539,041, the disclosure of which is herein incorporated by reference, the making of metallic powders by a process for reducing oxides, hydroxides or metal salts with the aid of polyols, is described. Particularly when starting with cobalt hydroxide, it is possible to obtain powders of metallic cobalt as essentially spherical, non-agglomerated particles. Further studies have shown in particular that it is possible to obtain non-agglomerated metallic powders having controlled average diameters of the particles, for instance by varying the .concentration of the starting hydroxide or metal salt, in relation to the polyol(s). Thus, in the case of cobalt, it is possible to obtain particles with an average diameter of, for instance 1, 2 or 3 .mu.m, by using the ratios cobalt hydroxide/polyol of 0.033, 0.1 or 0.340 g cobalt/cm.sup.3 polyol, respectively. Similarly, it is possible to obtain particles with adjustable average dimensions, smaller than 1 .mu.m by seeding the reaction mixture with the aid of very fine metallic particles (for instance palladium) either by adding a metal salt or hydroxide reacting more quickly than the cobalt salt or hydroxide with the polyol. This is particularly the case with silver salts, in particular silver nitrate, which are quickly reduced to metallic silver in the form of very fine particles of which the number is roughly proportional to the quantity of silver introduced into the reaction chamber. The silver or palladium particles so formed serve as seeds for the growth of cobalt particles which are subsequently formed by reduction of the cobalt hydroxide or salt by the polyol. The higher the number of seed particles, the smaller the dimensions of the final cobalt particles. For instance, when using a molar ratio silver/cobalt in the range of 10.sup.-4 -10.sup.-2, one can obtain cobalt particles having average dimensions that vary from 0.1 to 0.3 .mu.m and the range can be extended by varying this ratio between 10.sup.-5 and 10.sup.-1 for all the appropriate metals. These various methods for controlling the size of the metallic particles are particularly known and described by M. Figlarz et al, M.R.S. International Meeting on Advanced Materials, Vol. 3, Materials Research Society, pp. 125-140 (1989); F. Fievet et al, Solid State Ionics 32/33, 198-205 (1989); and F. Fievet et al, M.R.S Bulletin, December 1989, pp. 29-34.